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The science behind intake and exhaust valves

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For any internal combustion engine to run, the fuel and air mixture, called the charge, needs to enter the cylinder bore. After the combustion process for one firing event is complete, the inert exhaust gas must then exit the cylinder bore to make room for the sequence to start all over again. As long as the engine is running, these events happen continuously, in each cylinder bore.
The valve allows the charge to enter the bore, and also allows the exhaust to leave it. You can think of the valve as a gate on either the intake or exhaust port, operated by an intermediate component like the camshaft. The valve itself must withstand the heat from combustion, but also be light enough to open and close quickly and seal tightly; it needs to be durable, as well. And it has to do all of this at an extremely rapid rate for countless times over the life of the engine.
Types of Valves
In the early days of the four-stroke automobile engine, engineers explored different theories in valve design, shape and activation. Eventually, three different concepts were brought to market on production engines: sleeve, rotary and poppet valves. The sleeve and rotary designs were never widely embraced due to complexities in manufacturing, sealing and activation; the poppet valve, though inefficient in many ways, proved to be the best gatekeeper for the cylinder bore.
When equipped with a sleeve valve, the engine usually had the sleeve attached to either the cylinder head or to the engine block itself, alongside the bore. A sleeve-valve engine looked nothing like a conventional poppet-valve design, regardless of where the poppet valves were placed (in the cylinder head or in the engine block). There were two sleeves each for the intake and exhaust ports, with openings near the top of the cylinder bore. The openings in the companion sleeves needed to be aligned, exposing a port for either intake or exhaust functions. The sleeves were mechanically moved by a link to the camshaft. Though different in function and appearance, one can think of the reed valves used in a two-stroke engine as being related to the four-stroke sleeve design.
The rotary valve resembled a camshaft, but with timed open ports; the placement of the opening on the blank had to be linked in the proper sequence to the crankshaft to allow the engine to breathe. While promising because of its simplicity--few ancillary components were required (no camshaft, springs, rocker arms, etc.)--and the fact that it could be made to seal, the rotary valve proved hard to put into production. Lubricating oil control seals were an issue, as was the need for exacting tolerances to be effective against combustion pressure when closed.
The rotary valve never enjoyed much use in a production automobile engine, but in the late 1980s, a New Jersey-based company, Coates International, retrofitted many modern production engines with a rotary valve design with excellent results. It allowed for higher horsepower, lower emissions and better fuel economy than the same engine equipped with poppet valves. Still, to the best of my knowledge, even with Coates' advancements, Detroit's carmakers were so invested in poppet valve technology that the rotary valve system was once again rejected.
Poppet Valves
A poppet valve consists of a disc-shaped head with a conical seating surface; that surface attaches to a stem that acts as a guide surface for its up-and-down motion. Poppet valves have several advantages over other designs: They are self-centering as they close onto the seat surface; they possess the ability to rotate to a new position for improved wear characteristics; and they are relatively easy to service during the life of an engine.
For mechanical strength and to assist flow into and out of the cylinder bore, the valve stem is blended into the head portion, forming a neck with a generous radius. When exposed to cylinder pressure, the valve head may conform to the seating surface and negate some distortion. The valve stem is made with a working clearance so that it does not bind in the guide as it strokes back and forth. Due to the higher thermal loading, the exhaust valve stem enjoys slightly more guide clearance than the intake side.
The head diameter of the exhaust valve is typically less than that of the intake valve, because cylinder gases are more easily evacuated at exhaust pressure than at the intake pressure differential. Another reason for the difference in diameter is to reduce thermal loading by virtue of a shorter path of heat flow.
Face Angle
The conical face of the poppet valve makes an angle of either 45 or 30 degrees with the plane of the cylinder head. Although a 45-degree angle provides a higher seating pressure for a given valve spring load, a 30 degree angle promotes better flow at a given lift. In some engines, the face angles of the exhaust and intake valves may be 45 and 30 degrees, respectively. To improve the seating, the exhaust valves may be installed with a differential face-to-seat angle of up to one degree. The effect of this is initially to concentrate the seat bedding towards the larger diameter and combustion side of the valve face, so that any dishing of the valve head tends to centralize the seat bedding.
In many applications, the face angle of the valve may either match or have a slight positive interference with the seat angle. In general, the actual bedding area should be neither too narrow (which would hinder heat dissipation) nor too wide (which would reduce seating pressure and be less effective in breaking up deposits).
A Stressful Life
Poppet valves experience a tortured existence; they are exposed to severe mechanical and thermal stress. Exhaust valves, in particular, have to withstand mean operating temperatures that may approach 1,700 degrees Fahrenheit, transferring approximately 75 percent of this heat to the cooling system through the valve seat, with the remainder flowing through the valve guide.
Since thermal loading on the inlet valve is less severe, this valve is generally made from low-alloy silicon-chromium steel. This alloy, known as Silchrome, was invented in the U.S. in 1926; it combines 3 to 3.5 percent silicon with 8 to 9 percent chromium and 0.4 to 0.5 percent carbon. For very light-duty applications, a mix of 1 percent chromium steel may be used. A one-piece valve constructed of these materials would be locally hardened for tip, groove or seat-wear resistance, while the stem may be chromium plated.
For the exhaust valve, chromium-manganese-nickel steel is the most widely used material. This high-alloy material has a combination of hot strength and corrosion resistance that meets the requirements of most engines up to over 1,500 degrees F, constant thermal load. Sometimes, the durability of the exhaust valves is enhanced by applying an aluminum coating to the heads. General Motors was the first automaker to introduce this technique, using a process that formed a tough corrosion-resistant layer and improved heat conductivity.
Bi-metal two-piece exhaust valves are also common on production engines, because they are more economical to produce. A high-alloy steel only needs to be used for the portion of the cylinder head where the maximum hot strength is required. A less expensive low-alloy steel can be used for the stem, allowing for the best guide wear properties without the aid of chrome plating.
When a two-piece valve is created, the parts are joined by a process called friction welding. This is performed by the heat created when one of the parts being butted together is rotated against the other stationary part under an axial load; the final welding occurs when the rotation stops. Friction welding was originally used by the Germans during World War II to join plastic components together. After the war, American engineers modified the process to work on metals.
Improving Heat Transfer
During the 1930s, an American engineer, S.D. Heron, was one of the first to explore improving the heat transfer of the exhaust valve so that increased thermal loading (horsepower, in lay terms) could be created. Mercury-filled exhaust valves were first employed on farm lighting plant engines and lived a long life despite being exposed to blasts of over 1,800 degrees. But mercury had its limitations and was not the ideal material to use.
Heron then experimented with sodium-filled valve stems. Sodium was an excellent conductor of heat, was light in comparison to mercury, and was easier to seal into the stem and did not leak out over time. In addition, semi-metallic sodium transferred heat through convection as well as through conduction. Sodium-filled exhaust valves were usually found in engines that were exposed to severe loading for long periods of time, such as agricultural and industrial applications, marine applications, airplanes, trucks and racing engines.
Though there have been many advancements to engine design since the first poppet valve appeared in the early 1900s, not much has changed in how the charge enters and then exits the cylinder. It's a marvel that is often overlooked by even the most ardent engineers and automotive enthusiasts.

This article originally appeared in the July, 2010 issue of Hemmings Classic Car.